Aperiodic lattice lasers for terahertz-over-fibre systems

Abstract

Terahertz-over-Fibre (ToF) is a promising photonic technology for the next generation high-data-rate wireless communication. The key to a successful ToF system is an integrated, compact and powerful terahertz (THz) source. Quantum cascade lasers (QCLs) have this potential at a range of THz frequency bands. However, a major challenge for a high-capacity wireless scheme at this high frequency is to achieve a multitude of optical channels and also requiring spectral agility. This thesis investigates the ToF technology by exploiting a computer-generated hologram, here working as an aperiodic photonic lattice, integrated within a QCL, providing electronically switchable lasing modes at multiple, pre-defined THz frequencies. For the first time, a transfer matrix method (TMM) has been employed to study nonlinear optics, focusing on sum- and difference-frequency generation, within such aperiodic lattice (AL) microcavities. As a foundation, before performing the nonlinear calculations, the microcavity linear optical response is studied. The results reveal that these holographically designed microcavities possess a multi-band spectra, containing both defect and band-edge photonic states. Furthermore, under the influence of the gain, it has been observed that the lasing solutions of the defect modes are insensitive to variations of the grating coupling strength; this outcome is particularly useful, as exemplified elsewhere, in relation to the fabrication imperfections during the ion milling of photonic lattices within THz QCLs. Remarkable spectral control over a wide bandwidth is also maintained under the influence of cavity feedback when an AL is integrated within a Fabry-P´erot (FP) QCL. The introduction of the AL to FP cavities gives rise to: (i) electronic switching and (ii) mode-dependant electronic fine-tuning, as verified by numerical and previous experimental demonstrations. Specifically, TMM has been used to model the cavity pulling effect at multiple lasing modes by employing the Kramers-Kronig principle. Proceeding from THz to telecom wavelengths, a multimode of THz emission has been up-converted to the telecom carrier as experimentally demonstrated in FP QCLs. TMM study of this up-conversion process revealed the interference fringes of the background FP cavity, which significantly enhances the signal power. Analysis of the cavity dispersion shows that a single frequency near-infrared (NIR) laser can up-convert THz modes spanning a bandwidth of 220 GHz, only limited by the group-index mismatch between the NIR and THz waves. Finally, a numerical model is developed for the ToF system based upon the computer-generated hologram studied here. The results show an enhancement of the photonic density of mode (DOM) at designed THz frequencies, as expected; but crucially, it also caused an analogous enhancement of signal power in the NIR with a straightforward one-to-one correspondence between the signal spectra and the hologram response function. The photonic lattice dispersion study revealed that each THz mode has its own effective modal index, allowing us to engineer them to phase-match to different optical pump frequencies. As a result, an electronically switchable sum- and difference-frequency generation process, at optical wavelengths, has been demonstrated in an aperiodic lattice laser. The nonlinear TMM study of the THz-NIR up- conversion also reveals that the phase-matching process could be electronically tuned over the entire telecom band, along with the inherent multi-band hologram response at the THz band, within a single-selection laser. Compared to previously published work, where multiple double-metal QCL waveguides were needed, the integrated hologram technique could be a key breakthrough for tunable multi-channel THz wireless communications.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Matthew Halsall (Supervisor) & Subhasish Chakraborty (Supervisor)